This invention relates to solid state electroluminescent devices, and more particularly, such devices of a discrete light source nature employing light-emitting diodes.
In recent years, light-emitting diode (LED) devices of the type providing a discrete source of light (as compared with display devices such as commonly employed for alphabetic and numeric or so-called alphanumeric visual displays) have found increasing use as indicators, panel lights, and even as devices for illuminating or "backlighting" of objects or devices in proximity thereto, such as push buttons, meters, printed legends on electronics equipment and the like. Situations where "backlighting" is required include translucent panels, switches or other button-like actuators, equipment fascia or the like. Accordingly, such LED discrete devices have supplanted conventional incandescent light sources in many commercial applications.
As designers have found new ways to employ such discrete devices in myriad areas of technology and commercial equipment design, need has arisen for such devices having increased light emitting area and providing as well for emission of light across the entire width and height of the face of such device. Just as importantly, there has been a need for devices having a light emitting area of a geometry suitable for arraying while providing a pleasing and useful format. It is well recognized, in addition, that there is commercial desirability that such devices have greater intensity while at the same time providing for uniform emission of light across the entire face of such device.
A recent development of note has been the incorporation of LED's in linear arrays. In such arrays, individual discrete devices are closely spaced so as to provide a linear or bar-type visual display, that is, an essentially solid state bargraph meter. As will be understood, the individual devices may be successively energized or deenergized to provide a bar or column of light of variable length. Such bargraph configurations have been proposed for use as cockpit displays, for example. In such arrayed configurations, it is desirable that the illuminated lengths appear to be more or less solid and continuous in nature with relatively little spacing between adjacent light emitting areas of such device. Heretofore, it has been known to create monolithic arrays constituted by adjacent LED devices in a single substrate or crystal. Such configurations are costly and difficult to manufacture. Similar configurations can be achieved by simply placing individual discrete devices in side-by-side relationship.
In such linear arrays, as well as other arrayed configurations, it is usually desirable to prevent or minimize so-called optical cross-talk between adjacent discrete devices. Such cross-talk is manifested by light from an illuminated device being coupled into an adjacent unlighted device. The effect of such cross-talk is one of semaring or indefinite demarcation between adjacent lighted and unlighted devices. The resultant lack of contrast between the adjacent lighted and unlighted areas is generally undesirable.
Typically, LED devices of the discrete type employ one or more chips or so-called die of monocrystalline semiconductive material selected from binary, ternary or quaternary compounds of elements selected from groups III and V of the periodic table, such as Ga, In, Al, P and As. Conventionally, such die are produced by dicing a thin wafer of such a monocrystalline semiconductor, and typically comprises a first region of a first electrical conductivity type, such as n-type, and a thin further region of opposite conductivity type, such as p-type, so as to provide a p-n junction in the material providing a diode structure with an active region where light omission occurs through radiative recombinant phenomena.
Price competitive factors in the commercial marketing of LED discrete devices require that their manufacturing costs be minimized. Such considerations make it difficult to meet the objectives dictated by the foregoing criteria.
Most commercially available LED devices which emit light in the visible spectrum and which are useful for devices of the present character are of GaP and GaAsP. Such materials have different light emitting characteristics. For example, GaP characteristically has a more or less isotropic emission pattern, emitting light from the sides as well as the faces of such device due to the semitransparent nature of the crystalline material. In comparison, GaAsP devices emit light having a Lambertian emission characteristic such that light is predominently emitted in a direction which is normal to a face of such device, it being understood that the p-n junction is conventionally parallel to the light emitting face and is spaced at least several microns from the light-emitting face.